The present invention generally relates to self-attaching fasteners and, more specifically to clinch nuts and installation tooling therefor.
Self-attaching fasteners are used in many industries such as, for example, the automotive and appliance industries to secure various components to metal panels. When clinch nuts are attached to the metal panels, screws or bolts are threaded into the clinch nuts and tightened to prescribed torque values. During installation, the clinch nuts must have sufficient rotational resistance to keep them from rotating relative to the metal panels when the screws are inserted and tightened. During service, the clinch nuts must have sufficient pull-through resistance to keep them from pulling out of the metal panel when external forces such as, for example, vibration or other tensile forces are applied.
A clinch nut typically includes a central pilot or punch portion which at least partially extends into an opening in a metal plate or panel. When the clinch nut is self piercing, the central pilot portion cooperates with tooling to form the opening in the metal panel when attaching the clinch nut to the metal panel. The clinch nut is attached to the metal panel by a die member which forms a mechanical interlock between the clinch nut and the metal panel. The die member typically deforms the metal panel about the opening into an annular groove of the clinch nut which encircles the pilot portion and/or deforms the pilot portion of the clinch nut over the metal panel to entrap the metal panel.
For example, U.S. Pat. No. 3,053,300 discloses a clinch nut having a central pilot portion which extends through a pre-formed opening in a metal panel and is folded over to stake the periphery of the opening. The deformation of the central pilot forces the metal panel to conform to an undulating surface of the annular groove and to form the interlock between the clinch nut and metal panel. While this clinch nut may have a relatively high pull-out resistance, the deformation of the central pilot can easily distort the internal threads of the clinch nut.
One approach to eliminate distortion of the internal threads when deforming the pilot is to deform the metal panel to form the interlock rather than the pilot of the clinch nut. For example, U.S. Pat. Nos. 3,878,599 and 4,690,599 each disclose a clinch nut having an undercut on either the inner or outer wall of the groove. Material of the metal panel is forced into the undercut to improve the interlock formed between the clinch nut and the metal panel. With relatively thin metal panels, however, very little material is forced into the undercut, resulting in a relatively low pull-out resistance.
One approach to increase the pull-out resistance of clinch nuts of this type is to form a double-undercut groove. For example, U.S. Pat. No. 5,340,251 discloses a clinch nut having undercuts in both the inner and outer walls so that the annular groove is "dove-tail" shaped in cross section. The metal panel is forced into both of the undercuts to form an improved interlock between the clinch nut and metal panel. The deformation of the metal panel required to fill both undercuts, however, is difficult to obtain using conventional forming techniques, resulting in inconsistent pull-out resistance.
An additional problem with the above-noted self-clinching fasteners is that they typically to not function well with thin metal panels, that is, panels of 3 mm or less. Accordingly, there is a need in the art for an improved clinch nut which can be reliably and consistently attached to a thin metal panel having sufficient pull-out strength, sufficient rotational resistance, and without having distortion of the internal threads. Additionally, there is a need for an improved die member for installing a clinch nut in a thin metal panel having sufficient pull-out strength, having sufficient rotational resistance, and without having distortion of the internal threads. Furthermore, there is a need for both the clinch nut and the die member to be relatively inexpensive to produce and relatively easy to use.